1. Field
The present invention relates generally to data communication, and more specifically to techniques for allocating downlink resources in a multiple-input multiple-output (MIMO) communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on, for a number of users. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access techniques.
A multiple-input multiple-output (MIMO) communication system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for transmission of multiple independent data streams. In one common MIMO system implementation, the data streams are transmitted to a single terminal at any given time. However, a multiple access communication system having a base station with multiple antennas may also concurrently communicate with a number of terminals. In this case, the base station employs a number of antennas and each terminal employs NR antennas to receive one or more of the multiple data streams.
The connection between a multiple-antenna base station and a single multiple-antenna terminal is called a MIMO channel. A MIMO channel formed by these NT transmit and NR receive antennas may be decomposed into NC independent channels, with NC≦min {NT, NR}. Each of the NC independent channels is also referred to as a spatial subchannel of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities of these subchannels created by the multiple transmit and receive antennas are utilized.
Each MIMO channel between the base station and a terminal typically experiences different link characteristics and is associated with different transmission capability, so the spatial subchannels available to each terminal have different effective capacities. Efficient use of the available downlink resources (and higher throughput) may be achieved if the NC available spatial subchannels are effectively allocated such that data is transmitted on these subchannels to a “proper” set of terminals in the MIMO system.
There is therefore a need in the art for techniques to allocate downlink resources in a MIMO system to provide improved system performance.